Plan on doing more squadron painting, and was wondering if it's legal to use a single TIE/ln or Interceptor (using the rogues and villains single ship stands) to represent Howlrunner/Mauler Mithil or Soontir Fel. Or do they have to use the three on the tree with the same fighter model?

In practice I fly Vader as an Advanced flanked by 2 TIEs and no one has ever complained. As long as the cardboard is correct it's fine for most cases, but have the spares available in case someone gets shitty (and then never play that person again)

I'm not sure for tournament play. You'd want to check with whoever was running it. That said, the base and cardboard are the important parts rules wise, and just having one interceptor didn't obfuscate it. If I were your opponent I wouldn't mind at all.